1. Field of the Invention
The present invention relates to a technology for selecting a communications system to be used and switching from system to system among a plurality of different communications systems.
2. Description of the Related Art
Construction of communications infrastructure is in progress at present, and there are a plurality of systems such as W-CDMA (Wideband CDMA), CDMA2000 (Wideband CDMA one) as third generation mobile communication systems, which are becoming the mainstream among wireless communication systems. These communication systems allow high-speed and high-capacity communication; however the power consumption of mobile communication terminal user equipment (hereinafter referred to as user equipment) such as cellular phones is higher than user equipment of the conventional communication systems such as second generation mobile communication systems etc.
Currently, user equipment in general has a function for detecting the remaining battery level and, when the remaining battery level falls below a prescribed level, for notifying a user possessing the equipment of the low battery level by an alarm sound etc. However, in the case that the alarm sounds while the user equipment is in use, if it were during a voice call, for example, the call would be interrupted in several tens of seconds. In other words, although the user equipment has such a function, its alarm does not sound until the remaining battery level falls below the level insufficient to continue the call, and therefore, communication cannot be maintained with the user equipment. The user equipment, the call of which was terminated due to battery discharge, cannot be used for communication until its battery is recharged.
Technologies, which allow the battery power consumption of mobile communication user equipment to be controlled as soon as possible while maintaining communication quality to some extent, are proposed (for example, Patent Document 1). According to such technologies, the communication speed of mobile communication terminal user equipment is switched depending on the remaining battery power, and as the remaining battery level falls, the communication speed is reduced so as to extend the battery lifetime for communication.
There are other technologies proposed in which mobile communication terminal user equipment can selectively switch the communication system to use depending on the communication environment (Patent Documents 2, 3 and 4). According to such technologies, switching between W-CDMA and GSM (Global System for Mobile Communication) is performed, for example, in such a way that W-CDMA, which allows high-speed communication, is used when the remaining battery level is high, and can be switched to the battery-friendly GSM when the battery level is low.
Patent Document 1: Japanese laid-open disclosure public patent bulletin No. 2000-69107 (abstract, FIG. 4, and paragraph 0009)
Patent Document 2: Japanese laid-open disclosure public patent bulletin No. 011-252006 (abstract, FIG. 2, FIG. 4 and paragraph 0015)
Patent Document 3: Japanese laid-open disclosure public patent bulletin No. 2002-208870 (abstract, FIG. 1, and paragraphs 0038-0039)
Patent Document 4: Japanese laid-open disclosure public patent bulletin No. 2003-163966 (abstract, FIG. 1 and paragraph 0015)
As explained above, although one communication system among a plurality of communication systems can be selected and be switched to, the conventional art is not such that an optimum communication system can be automatically selected during a voice call. That is, in the conventional art, when the remaining battery level of the user equipment falls, the call has to be disconnected once, and the user is required to resume communication after switching communication systems to the one requiring less power consumption by manual operation. Such a case is troublesome in that the call has to be disconnected once and, in addition, resuming the communication disconnected once requires heavy battery consumption; therefore, efficiency in the use of the battery itself is reduced and, in some cases, the call cannot be resumed.
The user equipment described in the above Patent Document 2 comprises a function for selecting an optimum system in call request (at the beginning of a call). However, the user equipment only outputs an alarm sound when the battery level falls during a call.